1. Field of The Invention
The present invention relates to a process for concurrently fractionating and treating a full range fluid catalytically cracked naphtha stream. More particularly a selected boiling range fluid catalytically cracked naphtha stream is subjected to a process for the simultaneous thioetherification and splitting into a light boiling range naphtha, a medium boiling range naphtha and a heavy boiling range naphtha; and the selective hydrogenation of the dienes in the medium boiling range naphtha.
2. Related Information
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges which determine the compositions. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
Cracked naphtha as it comes from the catalytic cracker has a relatively high octane number as a result of the olefinic and aromatic compounds contained therein. In some cases this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane. Such cracked-stream sources such as from FCC, coker, visbreaker (and the like) typically contain around 90% of all of the “destination sulfur” that would have reported to refinery gasoline in the absence of all desulfurization treatment. The sulfur impurities require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations.
Hydrotreating is a broad term which includes saturation of olefins and aromatics and the reaction of organic nitrogen compounds to form ammonia. The reaction of organic sulfur compounds in a refinery stream with hydrogen over a catalyst to form H2S is typically called hydrodesulfurization. However hydrodesulfurization is included and is sometimes simply referred to as hydrotreating. The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:RSH+H2RH+H2S  (1)RCl+H2RH+HCl  (2)2RN+4H22RH+2NH3  (3)ROOH+2H2RH+2H2O  (4)
Typical operating conditions for naphtha HDS reactions are:
Temperature, ° F.450-650Pressure, psig250-750H2 recycle rate, SCF/bbl 700-2000Fresh H2 makeup, SCF/bbl150-500After the hydrotreating is complete, the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized naphtha.
In addition to supplying high octane blending components the cracked naphthas are often used as sources of olefins in other processes such as etherifications. The conditions of hydrotreating of the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction, thereby reducing the octane and causing a loss of source olefins.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. Since the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction the most common solution has been prefractionation prior to hydrotreating. The prefractionation produces a light boiling range naphtha (LCN) which boils in the range of C5 to about 250° F. and a heavy boiling range naphtha which boils in the range of from about 250-475° F. (HCN).
The predominant light or lower boiling sulfur compounds are mercaptans (RSH) while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. The separation by fractionation alone will not remove the mercaptans. However, in the past the mercaptans have been removed by oxidative processes involving caustic washing. A combination oxidative removal of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal of the mercaptans the mercaptans are converted to the corresponding disulfides.
In addition to treating the lighter portion of the naphtha to remove the mercaptans, it has been traditional to use the light portion as feed to a catalytic reforming unit to increase the octane number if necessary. Also the lighter fraction may be subjected to further separation to remove the valuable C5 olefins (amylenes) which are useful in preparing ethers.
U.S. Pat. No. 6,083,378 discloses a naphtha splitter as a distillation column reactor to treat a portion or all of the naphtha to remove the organic sulfur compounds contained therein. The catalyst is placed in the distillation column reactor such that the selected portion of the naphtha is contacted with the catalyst and treated. The catalyst may be placed in the rectification section to treat the lighter boiling range components only, in the stripping section to treat the heavier boiling range components only, or throughout the column to widely treat the naphtha. In addition the distillation column reactor may be combined with standard single pass fixed bed reactor(s) or another distillation column reactor to fine tune the treatment.
In hydrodesulfurizations it is known that H2S can recombine to form mercaptans thus increasing the amount of sulfur in the product. In U.S. Pat. No. 6,416,658 a full boiling range naphtha stream is subjected to simultaneous hydrodesulfurization and splitting into a light boiling range naphtha and a heavy boiling range naphtha followed by further hydrodesulfurization by contacting the light boiling range naphtha with hydrogen in countercurrent flow in a fixed bed of hydrodesulfurization catalyst to remove recombinant mercaptans which are formed by the reverse reaction of H2S with olefins in the naphtha during the initial hydrodesulfurization. In particular the entire recovered portion of the light naphtha from a reaction distillation column hydrodesulfurization is further contacted with hydrogen in countercurrent flow in a fixed bed of hydrodesulfurization catalyst.
It is an advantage of the present invention that the sulfur may be removed from the light naphtha portions of the stream without any substantial loss of the valuable lighter olefins. It is a particular advantage that recombinant mercaptans are not a concern in the present process of sulfur removal from the LCN. It is a further advantage that the dienes are reduced in the MCN.